FIG. 1 is a block diagram of a traditional analog DC offset correction loop 100 such as would be used around the baseband path of a direct conversion receiver or zero IF receiver of a radio, cell phone, or other communication device. Correction loop 100 is used for single ended applications and generally includes a baseband filter 102, an integrator 104, an operational transconductance amplifier (OTA) 106, and mixer 108 whose output impedance and DC bias current are represented by a current source 110 and a resistor 112. The desired DC voltage at the output of the baseband filter 102 is analog ground, Vag, however, the interaction of the current source 110, resistor 112, and the input referred DC offset of the baseband filter 102 generates an undesired DC offset at the filter input. The DC offset at the input of the baseband filter 102 is amplified through the baseband filter and produces a large offset at the baseband filter output. The integrator 104 and OTA 106 provide a feedback path to alter the current through the resistor 112 to adjust the voltage presented to the input of the baseband filter 102 thereby reducing the input referred DC offset.
Modern communications systems often require fast settling times. Even small DC offsets can saturate the baseband filter causing all linear loop equations to be invalid making it very difficult to settle the loop within the allotted time. Once the offset has been corrected, the correction loop must be moved to a much lower corner frequency or opened completely. Making a transition from a very wide offset correction loop bandwidth to a very narrow bandwidth poses a problem due to the transient response produced when making such a large transition. Opening the loop in an analog DC offset loop causes the correction voltage to drift from the desired value due to leakage on the integrator's 104 capacitor.
Accordingly, there is a need for an improved method and apparatus for correcting DC offsets, particularly those offsets which occur in zero IF and direct conversion receivers.